1. Field of the Invention
The present invention relates generally to a valve train for an internal combustion engine or the like and more specifically to a variable valve timing arrangement therefor.
2. Description of the Prior Art
In a known arrangement such as shown in FIGS. 1 and 2 of the present application, it has been proposed to operate a poppet valve, such as an inlet or exhaust valve of an internal combustion engine, via a rocker arm 1 which engages a cam 2 at one end and which is pivotally mounted on top of the stem 3 of the valve 4 at the other end. The upper surface of the rocket arm 1 is contoured and adapted to abut a lever 5. The point of abutment with the lever 5 defines the pivot or fulcrum point of the rocker arm. With this arrangement, as the cam 2 rotates, the rocker arm 1 is cammed to pivot about the fulcrum point defined by the aformentioned contact and induce the valve 4 to reciprocate. To vary the timing and degree of lift of the valve 4, a second cam 6 is provided and adapted to abut the lever 5. The second cam 6 is selectively rotated by a suitable motor or actuator or the like (not shown). Thus, if the second cam 6 is rotated in a direction to urge the lever 5 to rotate counter-clockwise (viz., downwardly as seen in the drawings) the degree of valve lift and the duration that the valve is open will be increased. Rotation of the cam which allows the lever to pivot in the clockwise direction (as seen in the drawings) reduces the valve lift and the duration for which the valve is open.
However, this arrangement has suffered from a number of drawbacks. That is to say, as the cam rotates a thrust acting in the longitudinal direction of the rocker arm tends to be imparted to the same and as the rocker arm is pivotally mounted on top of the valve stem, the valve stem is subject to a moment which tends to bend it. To compensate for this bending phenomenon, either the diameter of the valve stem has to be increased or a tappet 7 such as shown in FIG. 2 has to be installed. Both of these countermeasures tend to undesirably increase the mass and thus the inertia of the moving elements and in the case wherein the tappet 7 is provided, the moment which would otherwise tend to bend the valve stem tends to bias the tappet sideways against the wall of the cylinder in which it is disposed, inducing friction loss and rapid wear. Moreover, the surfaces of the rocker arm and the lever between which the fulcrum point is defined, tend to exhibit a high relative sliding velocity which induces rapid wear therebetween.
Yet another major drawback is encountered by this prior art arrangement. The drawback arises in that the return stroke of the rocker arm (viz., as the poppet valve closes) must be induced exclusively by the valve spring (or springs), as it is not possible to install a suitable spring for this purpose between the rocker arm and the lever due to the prohibitively complex relative motion therebetween. Thus, when the lever is allowed to rotate in the clockwise direction (to reduce valve lift) the fulcrum point defined between the lever and the rocker arm tends to move in the direction of the pivot point of the rocker arm 1 reducing the moment biasing the rocker arm into contact with the cam 2. Accordingly, the rocker arm 1 is not held on the cam with sufficient force and tends to bounce on the cam rather than smoothly following, same leading to the generation of noise, vibration and undesirable wear. This problem is further enhanced by the need to provide a suitable clearance between the valve stem and rocker arm to allow for thermal expansion etc.
For a complete disclosure of the arrangement described above, reference may be made to U.S. Pat. No. 3,413,965 which issued on Dec. 3, 1969 in the name of J. M. Gavasso.